Coated Diamond-Turned Replication Master And Associated Process

ABSTRACT

A coated diamond-turned replication master for manufacturing wafer level lens arrays includes a low-hardness diamond-turnable sheet, a mold surface formed in the low-hardness diamond-turnable sheet, and an oxidation-resistant coating formed on the mold surface for protecting same. The mold surface includes a plurality of optical forms, all of which cut therein by a single diamond cutting tool. A process for forming a coated diamond-turned replication master includes a step of forming a mold surface in a low-hardness diamond-turnable sheet using a single diamond cutting tool. The mold surface includes a plurality of optical forms, all of which are formed by the single diamond cutting tool. The process also includes, after the step of forming a plurality of optical forms, coating the mold surface with an oxidation-resistant coating.

RELATED APPLICATIONS

This application claim priority to U.S. Provisional Patent Application 61/897,163, filed Oct. 29, 2014, the disclosure of which is incorporated herein by reference.

BACKGROUND

A silicon wafer may be processed to include a plurality of optical sensors. Prior to cutting the wafer into individual sensors, one or more additional layers containing a plurality of optical components, e.g., lenses, may be combined (stacked) with the wafer to form a plurality of combined sensor/optic components. The plurality of optical components on each layer are manufactured using a metal replication master that is manufactured and used to create the desired plurality of optical components in a layer that corresponds to the optical sensors formed on the silicon wafer. The metal master is typically created by machining optical forms in a nickel-phosphorous (NiP) substrate using micro-milling diamond turning. NiP is used because it is durable, stable, and resistant to chemicals that make it well suited to subsequent cleaning during use within a plastic mastering process.

However, the high hardness level of NiP causes significant tool wear during the creation of the metal master, resulting in change of the optical form shape as more lens forms are machined. Since there may be thousands of lens forms on a single metal master, this change in optical form shape is highly undesirable. Sometimes the diamond tool wears to the extent that a new tool (or many tools) is used to machine all instances of the optical form within the metal master. However, using multiple tools to create the single metal master produces errors in lens form position (registration) and lens form depth (corresponding to maximum lens thickness) across the metal master, resulting in lower yields in products manufactured using the metal master.

The shape of an optical form machined into a replication master is determined in part by the optical component it is designed to replicate. In the case of a plurality of lenses manufactured by a process that includes a metal master, the metal master includes a respective plurality of corresponding lens forms.

SUMMARY

According to one aspect, a process for forming a coated diamond-turned replication master is provided. The process includes a step of forming a mold surface in a low-hardness diamond-turnable sheet using a single diamond cutting tool. The mold surface includes a plurality of optical forms, all of which are formed by the single diamond cutting tool. The process also includes, after the step of forming a plurality of optical forms, coating the mold surface with an oxidation-resistant coating.

According to another aspect, a low-hardness diamond-turnable coated diamond-turned replication master for manufacturing wafer level lens arrays is provided. The replication master includes a low-hardness diamond-turnable sheet, a mold surface formed in the low-hardness diamond-turnable sheet, and an oxidation-resistant coating formed on the mold surface for protecting same. The mold surface includes a plurality of optical forms, all of which cut therein by a single diamond cutting tool.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows one exemplary metal master, with a low-hardness diamond-turnable layer thereon, for manufacturing wafer level lens arrays, in an embodiment.

FIG. 2 shows one exemplary metal mastering process for creating a metal master used to manufacture wafer level lens arrays, in an embodiment.

FIG. 3 shows one image of an optical form created within a prior-art NiP metal master using a diamond cutting tool that has previously created approximately seven-hundred similar optical forms.

FIG. 4 shows one exemplary image of an optical form cut within the low-hardness diamond-turnable layer of FIG. 1 by the diamond cutting tool after cutting approximately four-thousand six-hundred similar forms within the layer.

FIG. 5 shows exemplary measurement of a first lens created from a first cut optical form of the metal master of FIG. 1.

FIG. 6 shows exemplary measurement of a second lens created from a last cut optical form of the metal master of FIG. 1.

FIG. 7 shows a graph illustrating a best-case registration of lenses created using a prior-art NiP metal master.

FIG. 8 shows one exemplary graph illustrating best-case registration of lenses created using the metal master of FIG. 1.

FIG. 9 is a histogram showing variation in optical form depth in a plurality of optical forms cut into the prior-art NiP metal master.

FIG. 10 shows one exemplary histogram illustrating variation in optical form depth across a plurality of optical forms cut into the metal master of FIG. 1.

FIG. 11 is a plot showing the residual error of optical forms in a prior-art NiP master as a function of radial position along a diameter of the optical form.

FIG. 12 is a plot showing the residual error of optical forms in an exemplary metal master as a function of radial position along a diameter of the optical form.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows one exemplary coated diamond-turned replication master 100 for manufacturing wafer level lens arrays. Coated diamond-turned replication master 100 includes a low-hardness diamond-turnable sheet 104. A low-hardness diamond-turnable sheet is a sheet having at least a surface formed of a low-hardness diamond-turnable material. In an embodiment of coated diamond-turned replication master 100, low-hardness diamond-turnable sheet 104 is formed by coating a substrate 102 with a layer of low-hardness diamond-turnable material. In a different embodiment of coated diamond-turned replication master 100 does not include substrate 102, such that low-hardness diamond-turnable sheet 104 is a free-standing sheet or plate formed entirely of low-hardness diamond-turnable material.

A plurality of optical forms 106 (and optionally other features) is milled into low-hardness diamond-turnable sheet 104 to create a mold surface 108 that is then coated with an oxidation-resistant coating 109.

Mold surface 108 includes at least surfaces of optical forms 106. Mold surface 108 may also include an uncut surface region 107 between adjacent optical forms 106. In an embodiment of coated diamond-turned replication master 100, oxidation-resistant coating 109 covers optical forms 106 and one or more uncut surface regions 107. In a different embodiment of coated diamond-turned replication master 100, oxidation-resistant coating 109 covers optical forms 106, but no part of an uncut surface regions 107. In different embodiment of coated diamond-turned replication master 100, oxidation-resistant coating 109 covers optical forms 106, and a portion of one or more uncut surface regions 107 adjoining an edge of an optical form 106.

In a typical embodiment of coated diamond-turned replication master 100, low-hardness diamond-turnable sheet 104 is formed of high-purity copper, and oxidation-resistant coating 109 is formed of amorphous NiP. The milling is performed, for example, by a diamond turning machine 120 and a diamond cutting tool 122. Features of coated diamond-turned replication master 100 are not necessarily drawn to scale and may be exaggerated for clarity of illustration. In an embodiment, a thickness 114 of low-hardness diamond-turnable sheet 104 is approximately one millimeter before milling.

Hundreds or thousands of optical forms 106 may be formed in low-hardness diamond-turnable sheet 104. In a one embodiment, optical forms 106 are arranged in one or more evenly-spaced two-dimensional grids (a.k.a. “tilings”) on sheet 104 to maximize die count, i.e., the number of forms 106 on sheet 104. Optical forms 106 are, for example, arranged in a square, triangular, or hexagonal grid. In an embodiment, the evenly-spaced two-dimensional grid matches a corresponding two-dimensional grid of image sensor die on a CMOS image sensor wafer, such that when the grids are aligned, an each image sensor die is aligned with an optical form.

In an embodiment, each of the plurality of optical forms 106 is identical. Herein, two optical forms 106 are considered to be identical if their respective shapes differ by less than relevant fabrication tolerances. In an embodiment, each of the optical forms 106 in low-hardness diamond-turnable sheet 104 has a depth that varies as a function of position within the form. For example, each optical form 106 in FIG. 1 is concave and radially symmetric with a maximum depth at the center of the form. In a different example, each optical form 106 has a radially-symmetric gull-wing shaped surface. Such an optical form is has a maximum depth at a non-zero radial distance from the center of the form.

Oxidation-resistant coating 109 has a thickness 119 between 0.5 μm and 3.0 μm. If coating 109 is formed of nickel or a material with similar absorption, coating 109 is opaque to visible and near-IR light: the imaginary part of nickel's refractive index (k) exceeds k=3 at visible and near-IR free-space wavelengths (0.4 μm<λ₀<2 μm), the skin depth δ=λ₀/(2 πk) is less than 100 nm. In an embodiment, thickness 119 is uniform, where thickness 119 is measured perpendicular to the mold surface 108. For example, thickness 119(2) is equal to thickness 119(1).

Mold surface 108 is used to create an array of optical elements that may be combined with a plurality of image sensors formed on a silicon wafer for example. Once combined, the silicon wafer and optical array thereon are singulated to form individual sensors with optics.

The use of low-hardness diamond-turnable sheet 104 reduces wear on diamond cutting tool 122, thereby (a) allowing coated diamond-turned replication master 100 to be formed using a single cutting tool, and (b) resulting in a higher yield of the combined sensor and optical element as compared to use of a conventional NiP metal master. Certain embodiments of coated diamond-turned replication master 100 include sheet 104 formed of a low-hardness diamond-turnable material, such as copper, with a higher thermal conductivity than NiP. Such embodiments may provide an additional advantage over prior-art NiP masters by facilitating cooling of plastic molded to the master surface.

FIG. 2 shows one exemplary process 200 for forming a coated diamond-turned replication master for creating replication master 100 of FIG. 1. Step 202 is optional. In an embodiment of process 200 that does not include optional step 202, sheet 104 is a free-standing sheet formed of a low-hardness diamond-turnable material.

In optional step 202, process 200 coats a substrate with a layer of low-hardness diamond-turnable material to form a low-hardness diamond-turnable sheet. In one example of step 202, all surfaces of substrate 102 are coated, via electroplating, with low-hardness diamond-turnable material having thickness 114 of approximately one millimeter to form low-hardness diamond-turnable sheet 104. Substrate 102 may be formed of a material such as steel, aluminum, Invar36, or other suitable material.

Low-hardness diamond-turnable sheet 104 may be formed of a low-hardness diamond-turnable material. In an embodiment, low-hardness diamond-turnable sheet 104 is formed of a high-purity copper, such as Alloy 101 OFE Copper by Sequoia Brass & Copper (Hayward, Calif., USA).

Herein, a “diamond-turnable” material refers to materials used to fabricate optical elements or molds using a diamond turning process. Diamond turnable materials include, but are not limited to, those listed in Chapter 41 (Table 2) of Handbook of Optics, Volume II (McGraw-Hill, 1995) and in Paul, Evans, et al., “Chemical Aspects of Tool Wear in Single Point Diamond Turning,” Precision Engineering, Vol.18, pp. 4-19, 1996. Herein, a “low-hardness” material refers to a material that is less hard than nickel, wherein hardness is characterized by an indentation hardness measurement method such as the Mohs scale, the Brinell Scale, or the Vickers hardness test. Table 1 of Paul, Evans, et al. lists diamond-turnable elements with Brinell microhardness values less than nickel, including the following nine: indium, tin, lead, zinc, magnesium, aluminum, silver, gold, and copper. Low-hardness diamond-turnable sheet 104 may be formed of any of the above-mentioned nine elements, alone or in combination as a homogenous alloy with a spatially uniform low hardness.

In step 204, process 200 forms a mold surface in a low-hardness diamond-turnable sheet using a single diamond cutting tool. The mold surface includes a plurality of optical forms, all of which are formed by the single diamond cutting tool. In one example of step 204, diamond turning machine 120 and diamond cutting tool 122 are used to form a plurality of optical forms 106 within low-hardness diamond-turnable sheet 104, which results in mold surface 108. Mold surface 108 includes optical forms 106. In an example of step 204, forming the mold surface includes creating a plurality of optical forms arranged in an evenly-spaced two-dimensional grid for maximizing die count. The specification of a “single diamond cutting tool” denotes at least that diamond cutting tool 122 is neither relapped, nor refurbished, nor replaced, during step 204.

Optical forms 106 each have a depth 116. Thickness 114 of low-hardness diamond-turnable sheet 104 exceeds depth 116. For example, thickness 114 may exceed depth 116 by more than 100 microns.

In step 206, process 200 cleans the mold surface. In one example of step 206, mold surface 108 is cleaned in an ultrasonic cleaning tank to remove debris and coolant/lubricant residue resulting from step 204.

In step 208, process 200 coats the mold surface with a thin coating formed of oxidation-resistant material. In one example of step 208, mold surface 108 is coated with oxidation-resistant coating 109. Oxidation-resistant coating 109 has thickness 119 and is formed of nickel, NiP, NiB (nickel-boron), or a copper-nickel alloy, for example.

In an embodiment of process 200, step 208 includes electrolessly plating the mold surface, as known in the art. In an embodiment of process 200, step 208 includes electroplating the mold surface, as known in the art. In an embodiment of process 200, step 208 includes coating the mold surface via a vacuum deposition method.

Oxidation-resistant coating 109 preserves the optical finish of mold surface 108 and prevents oxidation and/or damage to mold surface 108. Such damage and oxidation may result from cleaning during the plastic mastering process that forms the array of optical elements. In an embodiment, oxidation-resistant coating 109 remains intact throughout the plastic mastering process. For example, oxidation-resistant coating 109 formed of nickel or NiP may withstand the application of a surface release agent during a lens fabrication process. One example of a lens fabrication process is described in U.S. Pat. No. 8,599,301 to Dowski et al. In this context, surface release agents are also called mold release agents, release layers, and anti-adhesive coating agents.

If thickness 119 is too large, the surface roughness of oxidation-resistant coating 109 degrades the optical quality of lenses made therewith. If thickness 119 is too small, coverage is not uniform and may result in exposed regions of mold surface 108.

Since the low-hardness diamond-turnable material of sheet 104 induces less wear on diamond cutting tool 122 compared to a conventional NiP layer, variation between optical forms 106 across coated diamond-turned replication master 100 is reduced, thereby increasing quality and yield of resulting products. Reduced wear on diamond cutting tool 122 decreases manufacturing costs, as the cost of a new cutting tool is not trivial.

FIG. 3 shows one image 300 of an optical form created within a prior-art NiP metal master using a diamond cutting tool that has previously created approximately seven-hundred (700) similar optical forms. Specifically, image 300 shows a plurality of rings 302 that indicate wear in the diamond cutting tool.

FIG. 4 shows one image 400 of an optical form 106 cut within low-hardness diamond-turnable sheet 104, formed of copper, by diamond cutting tool 122 after cutting approximately four thousand six hundred (4,600) similar forms within sheet 104. As can be seen by comparing image 400 with image 300 of FIG. 3, the wear on diamond cutting tool 122 is significantly less, even after cutting a significantly greater number of optical forms 106, than the diamond cutting tool used to produce the optical form in the NiP master of image 300.

FIG. 5 shows a measurement of a first-cut optical form 106(1) of coated diamond-turned replication master 100 of FIG. 1. FIG. 6 shows a measurement of a last-cut optical form 106(N) of coated diamond-turned replication master 100, where N≈6,000. The measurements of FIGS. 5 and 6 were performed with a Zygo® NewView™ 600 optical profiler. The measured “rms” and “Ra” values refer to the root-mean-square deviation and the arithmetic average deviation, respectively, from the lens mold design profile of optical form 106. The root-mean-square deviation remains less than 10 nm after N cuts, which demonstrates decreased degradation of diamond cutting tool 122 compared to cuts on prior-art NiP masters. Images 502 and 602 are of a 40 μm×40 μm region aligned with the center of optical forms 106(1) and 106(N), respectively.

FIG. 7 shows a graph 700 illustrating a best-case registration of lenses created using a prior-art NiP metal master. The registration spread is characterized by a two standard deviations value of 2σ_(Ni)=0.89 μm: the lens registration error of 95% of the lenses is less than or equal to 2σ_(Ni). The registration spread may also be characterized by a three standard deviations value of 3σ_(Ni)=1.1 μm: the lens registration error of 99.6% of the lenses is less than or equal to 3σ_(Ni).

FIG. 8 shows one exemplary graph 800 illustrating best-case registration of lenses created using coated diamond-turned replication master 100 of FIG. 1. The registration spread is characterized by a two standard deviations value of 2σ_(Cu)=0.66 μm: the lens registration error of 95% of the lenses is less than or equal to 2σ_(Cu). The registration spread may also be characterized by a three standard deviations value of 3σ_(Cu)=0.85 μm: the lens registration error of 99.6% of the lenses is less than or equal to 3σ_(Cu).

A comparison of graph 800 to graph 700 illustrates that registration of lenses created from coated diamond-turned replication master 100 is improved over registration of lenses created from the prior-art NiP metal master. Registration is improved when using coated diamond-turned replication master 100 because all optical forms 106 may be cut with one and only one diamond cutting tool 122, thereby resulting in significantly reduced wear of diamond cutting tool 122 and not replacement thereof during the optical form cutting process, step 204 of FIG. 2.

FIG. 9 a histogram 900 showing variation in optical form depth in a plurality of optical forms cut into the prior-art NiP metal master. FIG. 10 is an exemplary histogram 1000, respectively, illustrating variation in optical form depth across a plurality of optical forms 106 cut into coated diamond-turned replication master 100 of FIG. 1. Comparing histogram 1000 with histogram 900 illustrates that the variation in depth across a plurality of optical forms cut into coated diamond-turned replication master 100 is significantly less than the variation in depth of similar optical forms cut into the NiP metal master of the prior art. This is quantified by the standard deviation of the distributions in histograms 900 and 1000, which are σ₉=1.22 μm and σ₁₀=0.287 μm, respectively. The distribution of histogram 900 contains isolated, distinct peaks, which may reflect the systematic differences between different diamond cutting tools used during the NiP master cutting process. In comparison, the distribution of histogram 1000 consists of a single peak.

The improved consistency of optical form depth in coated diamond-turned replication master 100 means that lenses formed from coated diamond-turned replication master 100 will have smaller form depth variation than lenses formed from a prior-art NiP metal master based on the same lens design.

FIG. 11 is a plot 1100 showing residual data sets 1102 of optical forms in a prior-art NiP master as a function of radial position along a diameter of the optical form. Each residual data set 1102(1-4) represents residual error from an optical form located in the center column of a different row of a master. For clarity of illustration, not all residual data sets 1102 are plotted in plot 1100, and of those that are plotted, not all are labeled with a reference numeral. Each residual data set 1102 has a peak-to-valley residual, which is the difference between its maximum and minimum residual values. The standard deviation of these peak-to-valley residuals is σ_(pv11)=0.057 μm.

At a given radial position, some residual data sets 1102 are positive, and some are negative. For example, at radial position r=−0.5 mm, residual data sets 1102(1) and 1102(2) are positive, while residual data sets 1102(3) and 1102(4) are negative. At r=−0.5 mm, the residual spread, the difference between the maximum and minimum residual value, is approximately Δ₁₁=0.25 microns. The maximum value of residual spread Δ₁₁ exceeds 0.30 microns, for example, at selected radius magnitudes greater than 0.5 mm. Because of the sign differences of residual data sets 1102 at a given radial position, some lenses made from a prior-art master with residual data sets 1102 will have a non-uniform focal length error: some lenses will have focal length that is too long, while others will have a focal length that is too short, compared to the design focal length. This makes correcting for the residual difficult compared to correcting for residuals with the same sign, e.g., all positive or all negative at a radial position.

FIG. 12 is a plot 1200 showing residual data sets 1202 of optical forms in an exemplary coated diamond-turned replication master 100 as a function of radial position along a diameter of the optical form. Each residual data set 1202 represents residual error from an optical form located in the center column of a different row of a master. For clarity of illustration, not all residual data sets 1202 are plotted in plot 1200, and of those that are plotted, not all are labeled with a reference numeral. Each residual data set 1202 has a peak-to-valley residual, which is the difference between its maximum and minimum residual values. The standard deviation of these peak-to-valley residuals is σ_(pv12)=0.025 μm, which his less than one-half the prior-art value σ_(pv11).

In addition to having a smaller σ_(pv) than an exemplary prior-art NiP master, optical forms 106 of coated diamond-turned replication master 100 have residual data sets 1202 that resemble each other significantly more than residual data sets 1102 (FIG. 11) of the prior art resemble each other. Whereas residual spread Δ₁₁ exceeds 0.30 μm in plot 1100 (a prior-art NiP master), residual spread Δ₁₂ of plot 1200 is at most Δ₁₂≈0.12 μm, for example, in the radius range 1220. Moreover, residual data sets 1202 each have a similar concave shape, which will result in all lenses fabricated from coated diamond-turned replication master 100 having a uniform focal length error: the focal length of all the lenses will either be too long, or all be too short. Such a uniform focal length error can be corrected with a spacer with a uniform thickness for all lenses. Such a solution is not feasible for lenses made from prior-art masters with residuals similar to those shown in plot 1100.

Changes may be made in the above methods and systems without departing from the scope hereof. It should thus be noted that the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method and system, which, as a matter of language, might be said to fall therebetween. 

What is claimed is:
 1. A process for forming a coated diamond-turned replication master, comprising the steps of: forming a mold surface in a low-hardness diamond-turnable sheet using a single diamond cutting tool, the mold surface comprising a plurality of optical forms, all of which are formed by the single diamond cutting tool; and coating the mold surface with an oxidation-resistant coating.
 2. The process of claim 1, further comprising, before the step of forming the mold surface, forming the sheet of low-hardness diamond turnable material having a thickness between 0.5 mm and 2 mm.
 3. The process of claim 1, the step of coating the mold surface comprising forming a layer of the oxidation-resistant coating having a thickness between 0.5 μm and 3 μm.
 4. The process of claim 1, the step of forming the mold surface comprising creating a plurality of identical optical forms.
 5. The process of claim 0, the step of forming the mold surface comprising creating a plurality of optical forms arranged in an evenly-spaced two-dimensional grid.
 6. The process of claim 5, the evenly-spaced two-dimensional grid matching a corresponding two-dimensional grid of image sensor die on a CMOS image sensor wafer.
 7. The process of claim 1, the step of coating the mold surface comprising one or both of electroplating plating the mold surface and electrolessly plating the mold surface.
 8. The process of claim 1, the step of coating the mold surface comprising vacuum-depositing the oxidation-resistant coating.
 9. A coated diamond-turned replication master for manufacturing wafer level lens arrays, comprising: a low-hardness diamond-turnable sheet; a mold surface formed in the low-hardness diamond-turnable sheet, the mold surface comprising a plurality of optical forms, all of which are cut therein by a single diamond cutting tool; and an oxidation-resistant coating formed on the mold surface for protecting same.
 10. The metal master of claim 9, the low-hardness diamond-turnable sheet including a substrate coated with a low-hardness diamond-turnable material.
 11. The metal master of claim 10, the substrate being formed of one of steel, aluminum, or Invar.
 12. The metal master of claim 9, the low-hardness diamond-turnable sheet being formed of at least one element selected from the group of elements consisting of indium, tin, lead, zinc, magnesium, aluminum, silver, gold, and copper.
 13. The metal master of claim 9, the plurality of optical forms having an optical form depth, and the low-hardness diamond-turnable sheet having a thickness exceeding the optical form depth by at least 100 μm.
 14. The metal master of claim 9, the oxidation-resistant coating having a thickness between 0.5 μm and 3 μm.
 15. The metal master of claim 9, the oxidation-resistant coating being formed of a material that includes one or more of nickel, nickel phosphorus, nickel boron or a copper-nickel alloy.
 16. The metal master of claim 9, the oxidation-resistant coating being formed of an amorphous material.
 17. The metal master of claim 9, the mold surface comprising a plurality of optical forms arranged in an evenly-spaced two-dimensional grid.
 18. The metal master of claim 17, the evenly-spaced two-dimensional grid matching a corresponding two-dimensional grid of image sensor die on a CMOS image sensor wafer.
 19. The metal master of claim 9, each of the plurality of optical forms being identical.
 20. The metal master of claim 9, each of the plurality of optical forms having a depth that varies as a function of position within the form.
 21. The metal master of claim 20, each of the plurality of optical forms having a maximum depth at a non-zero radial distance from the center of the form. 